The official word on COPACETIC/COPASETIC/COPESETIC is ORIGIN UNKNOWN. This is confirmed by the New Shorter OED, The Random House Historical Dictionary of Slang, Merriam-Webster Unabridged Dictionary, Dictionary of American Regional English (DARE), etc., etc.
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The Fact on File Encyclopedia of Word and Phrase Origins:

COPACETIC: The word is also spelled ‘copesetic, leading Partridge to suggest that this slang for ‘excellent, all right,’ or ‘all safe or all clear,’ as he defines it, is a combination of ‘cope’ and ‘antiseptic.’ But the expression is American and was largely confined to black speech when first recorded in the 1920s [actually 1919 according to OED]. Luther Bill ‘Bojangles’ Robinson may have invented ‘copacetic’ and the great tap dancer certainly did popularize it in his routines, giving the word wide currency. Robinson claimed he coined it when he was a shoeshine boy in Richmond. But a number of Southerners have testified that they heard the expression used by their parents or grandparents long before this. Another theory holds that ‘copecetic’ is from a Yiddish word meaning the same. It’s also spelled ‘kopasetic’ and ‘kopesetic.’
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The Barnhart Concise Dictionary of Etymology:

COPACETIC adjective: 1919 ‘copasetic’ very good, all right, American English. Said to have originated among southern blacks in the 1800s [no mention of Bojangles], of uncertain origin. The suggestion that ‘copacetic’ came from a Hebrew phrase such as “(ha)kol b’seder” all in order, or . . . . is not accepted among scholars of American English.
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Also see the discussion below from 'Ask the Wordwizard':

Posted - 08 Oct 1998

Copasetic, a word that seems to have entered English-language (proeprly American) slang in the 1910s, is now seen as coming from the Chinook jargon word 'copasenee', meaning 'everything is satisfactory', especially as it was originally used on the waterways of Washington state. There have been, unsurprisingly, a number of alternativer etymologies. Among them are the painfully contrived phr. 'the cop is on the settee', ie. the cop is not paying attention, which supposed elided into 'copacetic' and was supposedly used as such by US hoodlums; another suggestion,less contrived but totally vague is that it was once 'an unknown Italian word'; another is the Fr. ‘coupersetique’: itself from ‘couper’: to strike, and thus meaning striking or worth a strike; the most feasible of the 'also-rans' is the Yiddish phr. 'hakol b'seder', all is in order or, in an earlier form, 'kol b'tzedek': all with justice. However Jonathan Lighter, in the Random House Historical Dictionary of American Slang, dismisses all these and says 'ety. unknown'.

COPACETIC In the current issue of Comments on Etymology, Professor Gerald Cohen of the University of Missouri-Rolla has put forward a plausible suggestion for the origin of this most puzzling American term, first recorded in print from 1921, and which means "fine; all right". He points to French "copain(s) c'est épatant!", "buddy(s), that's great!". Many of the very early examples spell the word as variations on "copasetee", which may well have been the earliest pronunciation, the French phrase being abbreviated by losing all except the first vowel of the last word. He argues that the phrase may have been picked up by American soldiers in France during the First World War. Other expressions were certainly created at that time by mangling French terms, such as the British Tommy's "san fairy ann", from "ça ne fait rien", and "napoo" from "il n'y en a plus". The suggestion is seductive and intriguing, but in the absence of firm evidence - which may now never be forthcoming - it has to remain a hypothesis.
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Here are some examples from the OED. Note the one from 1919, which is two years earlier than Quinion’s claim of the earliest quote:

<1919 “‘As to looks I'd call him, as ye might say, real COPASETIC.’ Mrs. Lukins expressed this opinion solemnly . . . Its last word stood for nothing more than an indefinite depth of meaning.”—Man for Ages by I Bacheller, iv. page 69>

For anyone who has installed trim or done finish-work in interior woodworking(wdwrkng), this word will resonate with rich meaning: when you cope an inside-corner-joint, and it fits snugly, looking for all the world like a miter, it is "copacetic", for sure.
The aesthete's aesthetic's proven when his coped-joint's copacetic.

Last edited by gdwdwrkr on Tue Dec 25, 2018 3:54 pm, edited 1 time in total.

For anyone who has installed trim or done finish-work in interior woodworking, this word will resonate with rich meaning: when you cope an inside-corner-joint, and it fits snugly, looking for all the world like a miter, it is "copacetic", for sure.
The aesthete's aesthetic's proven when his coped-joint's copacetic.